This invention relates to waste heat recovery, and more particularly to a process and apparatus for the recovery of heat from high temperature gases.
Heat exchange is an important aspect of essentially all process operations whether at high or low temperature processing conditions. Economics normally dictate effective utilization of heat transfer equipment with respect to processing streams. Waste heat recovery generally relates to the recovery of heat over and above basic heat requirements, e.g. in steam generation equipment, there are normally a convection section disposed in the equipment whereat the temperature level is insufficient for steam generation but at a level where sensible heat is available for heating duty, such as preheating water to be passed to a steam drum. There are some processing operations where heat is available for recovery, but is not effectively recovered, if at all, e.g., the operation of a cupola.
In a typical foundry operation, coke, limestone and a metallic portion, such as pig and scrap iron are introduced through a charge door into a cupola. Cold blast air is introduced through tuyeres in the bottom to provide the combustion medium for the coke. Additional air is induced through the charge door by an exhaust fan. Afterburners located above the charge door provide a source of ignition for carbon monoxide leaving the bed and for providing heat for the cupola when the cupola is not in production. Air entering the cupola in the form of blast air, charge door air, and afterburner air is normally cold and is heated to operating temperature by consuming coke in the lower portion of the cupola.
Hot gases at a temperature of from about 1800.degree. to about 2200.degree. F. during the melting operation are withdrawn from the top of the cupola and are generally passed to vertically disposed water scrubber wherein the gas is cooled to a temperature of from 400.degree. to 500.degree. F. prior to introduction into a solids collector, e.g. an electrostatic precipitator or bag house. With direct water cooling and scrubbing, a large quantity of steam is produced which increases the volume of gas through the downstream equipment.
Heat recovery systems have been installed in a small number of plants in the form of either a recuperative or regenerative type of heat recovery systems. With a recuperative type, expensive high alloy heat exchanger is employed to cool the hot gas by heating the blast air. This type of heat exchanger is very expensive due to the high alloy construction needed to withstand the high metal temperatures (1800.degree. F. to 2200.degree. F.) and the large amount of heat transfer surface as a result of the poor heat transfer coefficient of hot gas to cool air. The recuperative type is subject to mechanical failures due to the frequent wide swings in temperatures from 500.degree. F.-2000.degree. F. which can occur as much as 14 times a day with swings ranging from ambient to 2000.degree. F. occurring with the daily startup and shutdown routine.
In the regenerative type, an expensive mesh wheel rotates and is alternately heated by hot gas and cooled by cool air. This type of heat exchanger is very large and is the source of much maintenance and plant shutdowns due to seal failures and corrosion where cold air condenses moisture and sulfur dioxide from the hot gas.
Both the recuperative and regenerative type of waste heat recovery systems effectively function only when the plant is at operating temperatures, i.e., 1800.degree. to 2200.degree. F. (gas temperature) and large amounts of blast air are needed. During idle time, when the afterburners are holding the cupola at around 500.degree. to 1300.degree. F. and no blast air is required, negligible heat is recovered. Idle time can amount to 8 hours per day or as much as 12 hours per day. Corresponding melting time would only be 8 hours or 4 hours with effective heat recovery time of 8 or 4 hours per day. Generally, such systems were limited to heat recovery necessary for preheating combustion air to reduce fuel requirements. Some process operations require gas fired auxiliary equipment since fuel oil firing produced a dirty or sooty exhaust gas which could not be tolerated by the process operation.
As briefly hereinabove indicated, heat exchangers have been used for various waste heat duty using the conventional heat transfer mediums. In U.S. Pat. No. 3,426,733, reference is made to the use of close looped systems for heat recovery utilizing heat transfer fluids, such as eutectic salt mixtures, aromatic heat transfer oils, tetrachlorobiphenyl compounds, and the like, however, indicating that such systems had inherent difficulties because such systems were closed loops. In U.S. Pat. No. 2,910,244, there is disclosed a process and apparatus for effecting an endothermic chemical reaction utilizing a molten salt mixture as an intermediate heat transfer medium.